The Geometry of Klein's Riemann Surface

A process for preparing benzoic acid by oxidizing toluene with a molecular oxygen containing gas in the liquid phase, which is characterized in that the oxidation is performed in the substantial absence of lower fatty acids and/or halogen compounds, and in the presence of a catalyst composed of component A which is a nickel compound, and component B which is a manganese compound, the total amount of nickel metal and manganese metal ranging from 0.003 to 0.5% by weight based on the total oxidation reaction mixture, and the weight ratio of the nickel metal to the manganese metal being from 99.8/0.2 to 60/40, when the oxidation is performed at a temperature between 135 DEG C. and 185 DEG C., and from 99.8/0.2 to 5/95 when the oxidation is performed at a temperature between 185 DEG and 235 DEG C.